YES it is adjustable, you need to turn the small centre flat piece in the centre of the 13mm nut.
Clockwise is normally harder, counter clockwise is softer, beware if it has not been turned for a very long time it can sometimes tear the seal and cause a leak, just go gentle, you will feel a definite stop(when at softest) when going softer do not force it past this, it has I beleive about 12 clicks from fully hard to fully soft.
tabetha